Process for separating higher hydrocarbons from a gas mixture

ABSTRACT

A process for separating higher hydrocarbons from a gas mixture containing the latter and lower-boiling components by rectificatory decomposition is described. The feed gas mixture (6) is partially condensed (7) and fed to a separation column (9). A bottom fraction (27), rich in higher hydrocarbons, is removed from the bottom of the separation column (9), and a top fraction (10), rich in lower-boiling components, is removed from the top of the separation column. The top fraction (10) is partially condensed (11) and the resultant condensate is used as reflux for the separation column (9). 
     Both the partial condensation of the feed gas mixture and the partial condensation of the top fraction are produced by indirect heat exchange (7, 11) with a refrigerant, which consists of several components and is conveyed in an external circuit (18).

SUMMARY OF THE INVENTION

The invention relates to a process for separating higher hydrocarbonsfrom a gas mixture containing the latter and lower-boiling components byrectification, in which process the gas mixture is partially condensedand fed to a separation column, at whose bottom a fraction rich inhigher hydrocarbons and at whose top a fraction rich in lower-boilingcomponents are drawn off; in this case the top fraction is partiallycondensed, and the condensate is fed to the top of the separation columnas reflux.

Such a process is known from EP-B-0 318 504. In the known process, thecold required to condense the feed gas and top fraction is madeavailable, on the one hand, by one or more refrigeration circuits and,on the other, by active pressure reduction of the feed gas or residualgas. The refrigeration circuits work at constant evaporation temperatureand, with heat exchange with a condensing feed gas or top gas mixture,cause relatively large temperature differences and thus energy losses.The turbines used to generate the extreme cold are not suitable for allprocesses. In particular, in the case of temperature fluctuations, forexample, they exhibit high wear because of unsteady process conditions.The previously known process, therefore, does not work completelysatisfactorily in economic terms and operates reliably only if certainboundary conditions are observed.

The object of the invention is to provide a process of the initiallymentioned type which works more economically, can be used more flexiblywith respect to boundary conditions and, in particular, is also suitedfor relatively widely fluctuating parameters of the gas mixture to beseparated.

This object is achieved by having the condensation of the gas mixtureand the condensation the top fraction be produced by indirect heatexchange with a refrigerant, which consists of several components and isconveyed in an external circuit.

Structuring the process in this way makes it possible to variable matchthe refrigerant temperature to the requirements imposed by thecomposition of the feed gas and products. Compared to a refrigerantcascade, for example, this makes both low equipment costs and low energylosses possible. Also, extreme cold can be generated at reasonableexpense, so that the process of the invention does not require apressure-reduction turbine. The drawbacks associated with turbines withrespect to flexibility are thus avoided.

The energy advantages of the process of the invention are surprisinglygreat. Not only compensate for extra costs caused by themulticomponent-refrigerant circuit, but overall produce a clear increasein the economic efficiency of the process. In addition, the possibleapplications of the process are extraordinarily flexible.

The separation column used in the process is generally only operated asan enrichment column, i.e., the partially condensed gas mixture is fedto the lower area of the column.

To improve the rectifying action of the separation column even further,it is advantageous to remove an intermediate fraction from theseparation column at an intermediate point. The intermediate friction isat least partially condensed in indirect heat exchange with therefrigerant and is returned to the separation column.

This heat exchange occurs at a temperature that lies between thetemperature level of the condensation of the feed gas mixture and thatof the condensation of the top fraction. The corresponding heatexchangers are preferably connected in series on the refrigerant side,so that optimum use of the sliding evaporation temperature curve of themulticomponent refrigerant is produced. As a result, the process can beoperated especially advantageously with respect to energy. Of course, itis also possible and in many cases also advantageous to remove severalsuch intermediate fractions in an analogous way and to feed them to anindirect heat exchange with the refrigerant.

In the process of the invention, it is further advantageous to separatecompressed refrigerant inside the external refrigeration circuit into agaseous fraction and a liquid fraction. The gaseous fraction is cooledin indirect heat exchange with the portion that remains gaseous duringthe condensation of the top fraction and in this connection is condensedand then conveyed for indirect heat exchange with the top fraction.

After compression, the refrigerant that remains gaseous is thus used inan especially advantageous way to transmit extreme cold to the topfraction of the separation column. This further improves the energybalance of the process.

In the heat exchange with the portion of the top fraction that remainsgaseous, the refrigerant is preferably not only completely condensed,but also supercooled, so as to have available, after pressure reductionthe largest possible portion in the liquid state. After compression, therefrigerant that remains liquid is also supercooled as much as possible.

Downstream from the heat exchanger, the entire refrigerant stream can berecombined to produce reflux. After the heat exchange with the topfraction, the refrigerant, generally supplemented by the refrigerantfraction that remains liquid after compression, is brought into heatexchange with the gas mixture to be separated and first, if necessary,brought into heat exchange with the intermediate fraction.

According to a further development of the inventive idea, the process iscarried out with a time-variable throughput and/or time-variablecomposition of the gas mixture to be separated.

Of course, each process is subject to time fluctuations, for example,when a unit is started and stopped. But, this further developmentpertains to changes with significantly shorter periods, in generalshorter than one hour, preferably in the minute range, which exhibit,for example, temperature fluctuations of about 3 K/min and/or 10%changes in load per minute. Such deviations from steady-state behaviorcan also be caused by preceding process steps. For example, if the gasmixture to be separated in this process comes from a periodicallyoperated apparatus, e.g., reversible reactors. In particular, if suchpreconditions exist, a process with generation of extreme cold byturbines (e.g., according to EP-B-0 318 504) would lead to very highwear of the turbines and thus would often experience shutdowns and highcosts for the unit, in particular due to production loss. The process ofthe invention can, however, tolerate such fluctuations, since themulticomponent-refrigerant circuit used is not subject to any such wearphenomena and, like the previously known processes, various coldtemperature levels can still be made available.

In the event that the process is carried out in such a unsteady way withrelatively short periods, conventional regulating processes are oftenpushed to their limits since they react too sluggishly. According to afurther development of the process of the invention, it is thereforeprovided that the throughput and/or the composition of the gas mixtureto be separated is measured and the throughput of the refrigerant isadjusted to the various condensation stages on the basis of thismeasured value.

The necessary adaptations to the cold balance are thus performed not byadjustment but by control. In this connection, certain parameters, whichcan be determined in advance only partially by theoreticalconsiderations, must enter into the calculation of the regulatedquantities. Moreover, experimental values are necessary, which must befound the first time a unit is started up by operating personnel. Sincethe throughput and/or composition fluctuations of the gas mixture to beseparated are generally periodic, these values can be found by testingand then firmly specified. Self-regulating units that optimize suchparameters automatically as well as during continuous operation are alsoconceivable.

In the case of relatively short-term fluctuations of the compositions ofthe feed, intermediate-product and product streams, which are causedeither indirectly, by throughputs of varying levels, or directly by thefeed gas that builds up accordingly, another problem arises with thepreviously known processes of this kind. As a rule, the aluminum-plateheat exchangers usually used withstand the resulting frequent andshort-term temperature fluctuations and the thus induced mechanicalstresses for only a very short time. Also, coiled heat exchangers withaluminum pipes, whose design is better suited for compensating forthermal longitudinal deformations, can become leaky over time.

According to another aspect of the invention, heat exchangers made of amaterial with high long-term stability against mechanical stresses are,therefore, preferably used for indirect heat exchange between the topfraction and the refrigerant. In this connection, high-grade steel ispreferably used. An embodiment of the heat exchanger in a coiled type ofstructure, i.e., with pipes arranged helically on concentric cylindersurfaces, is advantageous.

Similarly, it is advantageous for the indirect heat exchange between theportion that remains gaseous during the condensation of the top fractionand the gaseous fraction of the refrigerant and/or for indirect heatexchange (7) between gas mixture (6) to be separated and the refrigerantand/or for indirect heat exchange (24) between intermediate fraction(28) and the refrigerant each to use a heat exchanger, which is made ofa material with high long-term stability against mechanical stresses.

According to a variant of the invention, a plate heat exchanger,especially an aluminum-plate heat exchanger, can be used for indirectheat exchange (7) between the gas mixture (6) to be separated and therefrigerant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate embodiments of the process according to theinvention.

The invention as well as other details of the invention are nowexplained in more detail based on two embodiments, which are depicted inFIGS. 1 and 2 as procedural diagrams. They relate to a use of theprocess of the invention, in which its advantages are especiallyeffective, namely the preparation of a product gas from a C₃ or C₄dehydration. Such a gas contains, in addition to the higherhydrocarbons, more volatile portions, mainly hydrogen, but also smallerportions of water, carbon monoxide, carbon dioxide, nitrogen, C₂hydrocarbons, etc. The process steps of the invention are used toseparate the undesired lighter components, which is required for furtherprocessing of the C₃ or C₄ components.

DETAILED DESCRIPTION OF THE DRAWINGS

In the process of FIG. 1, the dehydration product gas is brought in vialine 1 and first subjected to pretreatment. After cooling by means of anexternal refrigerating unit in a heat exchanger 2 and subsequent phaseseparation in a separator 3, chlorine traces are removed in an HClreactor 4 from the portion that remains gaseous, and this portion isdried (5). The prepurified gas in line 6 now represents the gas mixtureto be separated for the process of the invention and is also designatedhere as feed gas. It contains, for example, 30 to 70% of the morevolatile components, which are to be separated. (The percentages relatehere and below basically to the molar portions.)

The feed gas in line 6 is cooled in heat exchanger 7, partiallycondensed (to 5 to 40%, preferably 10 to 30%) and fed via line 8 abovethe bottom into a separation column 9. At the bottom of the separationcolumn, the desired higher hydrocarbons accumulate as a bottom product,are drawn off via line 27 and heated in heat exchanger 23. Together withthe higher-boiling components already condensed out during pretreatmentfrom separator 3, they are fed via line 32 for further treatment, forexample, to a depropanizer.

Line 10 conveys the top fraction of the separation column to a heatexchanger 11, in which the fraction is partially condensed. Thetwo-phase mixture is conveyed via line 12 to a separator 13, which isintegrated into the separation column. However, a phase separationdevice made as a separate component could also be used. The liquid fromthe separator flows as reflux into the separation column; the portion ofthe top fraction that remains gaseous is removed via a residual gas line14 and heated in heat exchanger 15 to approximately ambient temperature.This gas can be fed partially or completely via line 17 to a compressorunit and then to another preparation step, for example, pressure-swingadsorption. As an alternative or in parallel to this, residual gaseither is removed via line 16 and used, for example, as combustible gasor to regenerate dryer 5.

According to the invention, the cold required for feed gas condensation(heat exchanger 7) and top fraction condensation (heat exchanger 11) isgenerated by a multicomponent-refrigerant circuit 18 in which, in aknown way, a refrigerant is compressed and partially liquefied. Therefrigerant contains, for example, C₂ H₄, C₂ H₆, Iso-C₄ H₁₀ and someCH₄. The exact composition is determined based on the plots of therespective evaporation curves. Here, exact matching to the evaporationproperties of feed and intermediate product streams in their respectivespecial compositions is possible.

Compressed refrigerant is introduced as a two-phase mixture to arefrigerant separator 19. The gaseous portion (line 20) is condensed torecover extreme coldness in indirect heat exchange 15 with portion 14 ofthe top fraction that remains gaseous and supercooled. The temperatureof the refrigerant stream should be as low as possible, so that alsowith subsequent pressure reduction in flow-control valve 25, allrefrigerant remains liquid. As a result, with subsequent heat exchange11 with top fraction 10, a maximum amount of latent heat can beconverted.

Portion 21 of the refrigerant that remains liquid from refrigerantseparator 19 is also supercooled, namely in heat exchanger 22 againstrefrigerant under low pressure and in heat exchanger 23 against C₃₊ /C₄₊product stream 27 from the bottom of separation column 9 and againagainst low-pressure refrigerant. A first part of supercooled liquid issubjected to pressure reduction in flow-control valve 26a, with whichthe refrigerant portion that remains gaseous in separator 19 iscombined, heated in heat exchangers 24, 7 and 22, and again compressed.A second part is subjected to pressure reduction in 26b, heated in thelower part of heat exchanger 23 and then combined upstream from heatexchanger 7 with the other low-pressure refrigerant.

To improve the energy balance of the process further, an intermediatefraction 28 is brought out, in the embodiment, from separation column 9,partially condensed in heat exchange 24 with refrigerant, and fed backvia line 29 to separation column 9. Analogously, several suchintermediate fractions can also removed at various points for partialcondensation. This must be decided in the individual case based onweighing the higher expense in equipment, on the one hand, against thereduced energy losses, on the other.

The heat exchangers required in the embodiment are preferably producedas coiled equipment with pipes-made of high-grade steel.

According to one aspect of the invention, the process works with acontrol device instead of a regulating device, as is otherwise usuallydone. To do this, the flow of the gas mixture to be separated in line 6is measured (30). From this measured value, set points for therefrigeration requirement are found in a control unit 31 by means ofadditional parameters, which were calculated partly theoretically,partly based on experience, and then the flow in the refrigerant linesis adjusted. This manipulation is accomplished by controllingpressure-reduction valves 25, 26a, 26b.

The following numerical example relates to the separation of C₄hydrocarbons from the product gas of C₄ dehydration. Because of theintermittent operation, the dehydration reactors alternate thethroughput and composition of the product gas with an approximatelyfour-minute period. For each content, two values are indicated: on theleft for the phase of maximum throughput of the gas mixture to beseparated (612 mol/s via line 6) and the associated smaller relative,but larger absolute hydrogen portion (about 55%, corresponds to 334mol/s); on the right for minimum throughput (423 mol/s) and largerrelative, but lower absolute hydrogen content (about 64%, corresponds to275 mol/s).

The various streams, for which data are indicated in the table, arecharacterized by capital letters A to G. In detail, they mean:

A Feed gas before partial condensation (line 6)

B feed gas after partial condensation (line 8)

C bottom product (line 27)

D top fraction before partial condensation (line 10)

E top fraction after partial condensation (line 12)

F intermediate fraction before partial condensation (line 28)

G intermediate fraction after partial condensation (line 29)

In this special application, the refrigerant exhibits the followingmolar composition:

    ______________________________________                                        CH.sub.4                   2%                                                 C.sub.2 H.sub.4            20%                                                C.sub.2 H.sub.6            25%                                                Iso-C.sub.4 H.sub.10       53%                                                ______________________________________                                    

                  TABLE                                                           ______________________________________                                        Site Content/Component                                                                           at Maximum Load                                                                           at Minimum Load                                ______________________________________                                        A    H.sub.2           54.6  mol % 63.8   mol %                                    CH.sub.4          12.7  mol % 3.8    mol %                                    C.sub.2 H.sub.4   0.9   mol % 0.3    mol %                                    C.sub.2 H.sub.6   0.9   mol % 0.4    mol %                                    C.sub.3 H.sub.6   6.9   mol % 4.0    mol %                                    C.sub.3 H.sub.8   4.3   mol % 4.9    mol %                                    iso-C.sub.4 H.sub.10                                                                            5.8   mol % 8.5    mol %                                    isobutene         8.0   mol % 6.2    mol %                                    C.sub.4 H.sub.10  0.1   mol % 0.1    mol %                                    1-butene          0.4   mol % 0.3    mol %                                    1,2-butadiene     0.05  mol % 0.05   mol %                                    N.sub.2           4.0   mol % 5.7    mol %                                    CO                0.9   mol % 1.3    mol %                                    CO.sub.2          0.45  mol % 0.6    mol %                                    Pressure          11.0  bar   11.0   bar                                      Temperature       280.2 K.    280.2  K.                                  B    Pressure          10.7  bar   10.7   bar                                      Temperature       243.2 K.    243.2  K.                                       Liquid Portion    18.5%       17.5%                                      C    H.sub.2           0.5   mol % 0.6    mol %                                    CH.sub.4          1.4   mol % 0.4    mol %                                    C.sub.2 H.sub.4   0.6   mol % 0.2    mol %                                    C.sub.2 H.sub.6   1.0   mol % 0.4    mol %                                    C.sub.3 H.sub.6   21.6  mol % 12.7   mol %                                    C.sub.3 H.sub.8   15.2  mol % 18.2   mol %                                    iso-C.sub.4 H.sub.10                                                                            24.2  mol % 37.5   mol %                                    isobutene         33.0  mol % 27.5   mol %                                    C.sub.4 H.sub.10  0.4   mol % 0.4    mol %                                    1-butene          1.5   mol % 1.5    mol %                                    1,2-butadiene     0.2   mol % 0.2    mol %                                    N.sub.2           0.1   mol % 0.1    mol %                                    CO                <0.01 mol % <0.01  mol %                                    CO.sub.2          0.2   mol % 0.3    mol %                                    Pressure          10.7  bar   10.7   bar                                      Temperature       241.9 K.    242.0  K.                                  D    H.sub.2           67.9  mol % 78.4   mol %                                    CH.sub.4          15.5  mol % 4.6    mol %                                    C.sub.2 H.sub.4   1.1   mol % 0.4    mol %                                    C.sub.2 H.sub.6   1.0   mol % 0.5    mol %                                    C.sub.3 H.sub.6   5.4   mol % 3.6    mol %                                    C.sub.3 H.sub.8   2.4   mol % 3.1    mol %                                    iso-C.sub.4 H.sub.10                                                                            0.02  mol % 0.03   mol %                                    isobutene         <0.01 mol % <0.01  mol %                                    C.sub.4 H.sub.10  <0.01 mol % <0.01  mol %                                    1-butene          <0.01 mol % <0.01  mol %                                    1,2-butadiene     <0.01 mol % <0.01  mol %                                    N.sub.2           5.0   mol % 7.0    mol %                                    CO                1.1   mol % 1.6    mol %                                    CO.sub.2          0.5   mol % 0.8    mol %                                    Pressure          10.5  bar   10.5   bar                                      Temperature       221.0 K.    218.7  K.                                  E    Pressure          10.4  bar   10.4   bar                                      Temperature       203.3 K.    198.9  K.                                       Liquid Portion    5.4%        4.8%                                       F    H.sub.2           65.5  mol % 75.6   mol %                                    CH.sub.4          15.0  mol % 4.4    mol %                                    C.sub.2 H.sub.4   1.0   mol % 0.4    mol %                                    C.sub.2 H.sub.6   1.0   mol % 0.4    mol %                                    C.sub.3 H.sub.6   6.9   mol % 4.5    mol %                                    C.sub.3 H.sub.8   3.8   mol % 5.0    mol %                                    iso-C.sub.4 H.sub.10                                                                            0.3   mol % 0.4    mol %                                    isobutene         0.2   mol % 0.2    mol %                                    C.sub.4 H.sub.10  <0.01 mol % <0.01  mol %                                    1-butene          <0.01 mol % <0.01  mol %                                    1,2-butadiene     <0.01 mol % <0.01  mol %                                    N.sub.2           4.8   mol % 6.7    mol %                                    CO                1.1   mol % 1.5    mol %                                    CO.sub.2          0.5   mol % 0.7    mol %                                    Pressure          10.6  bar   10.6   bar                                      Temperature       232.0 K.    231.7  K.                                  G    Pressure          10.6  bars  10.6   bars                                     Temperature       223.2 K.    221.2  K.                                       Liquid Portion    3.6%        3.6%                                       ______________________________________                                    

The diagram of FIG. 2 shows another embodiment of the process of theinvention, which also is used preferably for preparing a product gasfrom C₃ or C₄ dehydration. Process steps and devices corresponding toone another carry the same reference numbers in both drawings.

Dehydration product gas is brought in via line 1 and subjected topretreatment similar to that of the process of FIG. 1 (cooling by meansof external refrigeration in heat exchanger 2, phase separation inseparator 3, removal of chlorine in HCl reactor 4, drying 5). The feedgas in line 6 is cooled in heat exchanger 7 and partially condensed. Thetwo-phase mixture is fed in via line 8 above the bottom of separationcolumn 9. At the bottom of the separation column, the desired higherhydrocarbons accumulate as bottom product, and they are drawn off vialine 27 and heated in heat exchanger 7'. They are removed separatelyfrom the higher-boiling components already condensed out in thepretreatment in separator 3.

Line 10 conveys the top fraction of the separation column to a heatexchanger 11, in which the fraction is partially condensed. Thetwo-phase mixture is conveyed via line 12 to a separator 13 located inthe upper area of the separation column. The portion of the top fractionthat remains gaseous is removed via a residual-gas line 14 and heated inheat exchanger 15 to approximately ambient temperature. This gas can bedrawn off via line 16 (for example to regenerate dryer 5) and/or vialine 17.

According to the invention, the cold required to condense the feed gas(heat exchanger 7) and top fraction (heat exchanger 11) is generated ina way similar to the process of FIG. 1 by a multicomponent-refrigerantcircuit 18.

The gaseous portion of the compressed refrigerant introduced intorefrigerant separator 19 (line 20) is condensed, to recover the extremecold by indirect heat exchange 15 with portion 14 of the top fractionthat remains gaseous, and is supercooled. This steam is then subjectedto pressure reduction in flow-control valve 25 and brought into indirectheat exchange 11 with top fraction 10 from separation column 9.

Liquefied portion 21 of refrigerant from refrigerant separator 19 issupercooled in heat exchanger 7'. The supercooled liquid is subjected topressure reduction in flow-control valve 26, combined with therefrigerant portion that remains gaseous in separator 19, heated in heatexchanger 7' and completely evaporated and then again compressed.

To reduce the investment costs of the unit, the intermediate coolingstep represented in FIG. 1 was eliminated in the process of FIG. 2. Heatexchanger 7' is embodied as a plate heat exchanger in this variant. Itcombines the function of heat exchangers 7, 22 and 23 of FIG. 1.

Control in the process of FIG. 2 is accomplished in a way similar tothat described above in FIG. 1. In this respect, measuring devices areprovided for the flow of gas mixture (30) to be separated in line 6 andfor the pressure of refrigerant (33) in line 20. The measured values areconverted in a control unit 31 to set points for the refrigerationrequirement. Then, the flow in the refrigerant lines (pressure-reductionvalves 25, 26) is adjusted.

The numerical examples from the above table also apply to the variant ofFIG. 2. The elimination of the intermediate cooling (heat exchanger 24of FIG. 1) produces only slight changes in the parameters of the otherstreams.

I claim:
 1. A process for separating higher-boiling hydrocarbons from agas mixture containing said higher-boiling hydrocarbons andlower-boiling components by rectification, said processcomprising:partially condensing said gas mixture and feeding thepartially condensed gas mixture to a separation column; removing abottom fraction, rich in said higher-boiling hydrocarbons, and a topfraction, rich in said lower-boiling components, from said separationcolumn; partially condensing said top fraction to form a condensate, andintroducing said condensate as reflux to the top of said separationcolumn; wherein partial condensation of said gas mixture and partialcondensation of said top fraction are achieved by indirect heat exchangewith a multicomponent refrigerant conveyed in an external refrigerationcircuit; and wherein said refrigerant is compressed and separated insaid external refrigeration circuit into a gaseous refrigerant fractionand a liquid refrigerant fraction, said gaseous refrigerant fraction iscooled and condensed by indirect heat exchange with the portion of saidtop fraction that remains gaseous after said partial condensation ofsaid top fraction and condensed gaseous refrigerant fraction is thensubjected to said indirect heat exchange with said top fraction whereinsaid top fraction is partially condensed.
 2. A process according toclaim 1, further comprising removing an intermediate fraction from anintermediate point of said separation column, at least partiallycondensing said intermediate fraction by indirect heat exchange withsaid refrigerant, and returning the at least partially condensedintermediate fraction to said separation column.
 3. A process accordingto claim 1, wherein the process is conducted using a time-variablethroughput, a time-variable composition of said gas mixture, or both. 4.A process according to claim 3, wherein the throughput, the compositionof said gas mixture, or both, is measured and the throughput ofrefrigerant in at least one of said indirect heat exchanges for partialcondensation is adjusted on the basis of the measurement.
 5. A processaccording to claim 1, wherein a heat exchanger having helically arrangedpipes is used for said indirect heat exchange between said top fractionand said refrigerant wherein said top fraction is partially condensed.6. A process according to claim 1, wherein a heat exchanger havinghelically arranged pipes is used for said indirect heat exchange betweensaid gaseous refrigerant fraction and said portion of said top fractionthat remains gaseous after said condensation of said top fraction.
 7. Aprocess according to claim 1, wherein a heat exchanger having helicallyarranged pipes is used for said indirect heat exchange between said gasmixture and said refrigerant.
 8. A process according to claim 1, whereina plate heat exchanger is used for said indirect heat exchange betweensaid gas mixture to be separated and said refrigerant.
 9. A processaccording to claim 8, wherein an aluminum-plate heat exchanger is usedfor said indirect heat exchange between said gas mixture and saidrefrigerant.
 10. A process according to claim 2, wherein a heat changerhaving helically arranged pipes is used for said indirect heat exchangebetween said intermediate fraction and said refrigerant.
 11. A processaccording to claim 2, wherein the process is conducted using atime-variable throughput, a time-variable composition of feed gasmixture to be separated, or both.
 12. A process according to claim 11,wherein the throughput, the composition of said gas mixture, or both, ismeasured and the throughput of refrigerant in at least one of saidindirect heat exchanges for partial condensation is adjusted on thebasis of the measurement.
 13. A process according to claim 2, wherein aheat exchanger having helically arranged pipes is used for said indirectheat exchange between said top fraction and said refrigerant whereinsaid top fraction is partially condensed.
 14. A process according toclaim 3, wherein a heat exchanger having helically arranged pipes isused for said indirect heat exchange between said top fraction and saidrefrigerant wherein said top fraction is partially condensed.
 15. Aprocess according to claim 4, wherein a heat exchanger having helicallyarranged pipes is used for said indirect heat exchange between said topfraction and said refrigerant wherein said top fraction is partiallycondensed.
 16. A process according to claim 11, wherein a heat exchangerhaving helically arranged pipes is used for said indirect heat exchangebetween said top fraction and said refrigerant wherein said top fractionis partially condensed.
 17. A process according to claim 12, wherein aheat exchanger having helically arranged pipes is used for said indirectheat exchange between said top fraction and said refrigerant whereinsaid top fraction is partially condensed.
 18. A process according toclaim 1, wherein said higher-boiling hydrocarbons are C₃₊ /C₄₊hydrocarbons and said lower boiling components comprise hydrogen andmethane.
 19. A process according to claim 1, wherein saidmulti-component refrigerant contains CH₃, C₂ H₄, C₂ H₆ and iso-C₄ H₁₀.20. A process according to claim 18, wherein said multi-componentrefrigerant contains CH₃, C₂ H₄, C₂ H₆ and iso-C₄ H₁₀.
 21. A processaccording to claim 1, wherein said gas mixture contains 30-70 mole % ofsaid lower-boiling component.
 22. A process according to claim 1,wherein 10-30 mole % of said gas mixture is condensed by said indirectheat exchange with said refrigerant.
 23. A process according to claim 1,wherein, after said partial condensation of said top fraction to formsaid condensate, the partially condensed top fraction is conveyed to aseparator from which said condensate is delivered to said separationcolumn as reflux and said portion of said top fraction that remainsgaseous is removed and subjected to said indirect heat exchange withsaid condensed gaseous refrigerant fraction.
 24. A process according toclaim 2, wherein said condensed gaseous refrigerant fraction, afterbeing subjected to indirect heat exchange with said top fraction, iscombined with a portion of said liquid refrigerant fraction and theresultant refrigerant fraction is subjected to said indirect heatexchange with said intermediate fraction wherein said intermediatefraction is at least partially condensed.
 25. A process according toclaim 24, wherein, after said indirect heat exchange with saidintermediate fraction, said resultant refrigerant fraction is combinedwith a further portion of said liquid refrigerant fraction and thecombined refrigerant stream is subjected to said indirect heat exchangewith said gas mixture wherein said gas mixture is partially condensed.26. A process according to claim 25, wherein, after said indirect heatexchange with said gas mixture, said combined refrigerant stream issubjected to indirect heat exchange with said liquid refrigerantfraction.
 27. A process according to claim 1, wherein said liquidrefrigerant fraction is subjected to indirect heat exchange with saidbottom fraction.
 28. A process according to claim 27, wherein a portionof said liquid refrigerant fraction, after undergoing said indirect heatexchange with said bottom fraction, is combined with said condensedgaseous refrigerant fraction at a point upstream of said indirect heatexchange between said top fraction and said condensed gaseousrefrigerant fraction wherein said top fraction is partially condensed.29. A process according to claim 2, wherein said condensed gaseousrefrigerant fraction, after being subjected to said indirect heatexchange with said top fraction, is combined with a portion of saidliquid refrigerant fraction and the resultant refrigerant fraction issubjected to said indirect heat exchange with said gas fraction andindirect heat exchange with said liquid refrigerant fraction.
 30. Aprocess for separating higher-boiling hydrocarbons from a gas mixturecontaining said higher-boiling hydrocarbons and lower-boiling componentsby rectification, said process comprising:subjecting said gas mixture toindirect heat exchange whereby a portion of said gas mixture iscondensed and feeding the resultant combined condensed and gaseousportions of said gas mixture to a separation column; removing a bottomfraction, rich in said higher-boiling hydrocarbons, and a top fraction,rich in said lower-boiling components, from said separation column;subjecting said top fraction to indirect heat exchange whereby a portionof said top fraction is condensed to form a condensate, and introducingsaid condensate as reflux to the top of said separation column; whereincondensation of said portion of said gas mixture and condensation ofsaid portion of said top fraction are achieved by indirect heat exchangewith a multicomponent refrigerant conveyed in an external refrigerationcircuit; and wherein said refrigerant is compressed and separated insaid external refrigeration circuit into a gaseous refrigerant fractionand a liquid refrigerant fraction, said gaseous refrigerant fraction iscooled and condensed by indirect heat exchange with the portion of saidtop fraction that remains gaseous after said condensation of a portionof said top fraction and condensed gaseous refrigerant fraction is thensubjected to said indirect heat exchange with said top fraction whereina portion of maid top fraction is condensed.
 31. A process according toclaim 30, wherein, after condensation of said portion of said topfraction, said top fraction is conveyed to a separator from which saidcondensate is delivered to said separation column as reflux and saidportion of said top fraction that remains gaseous is removed andsubjected to said indirect heat exchange with said condensed gaseousrefrigerant fraction.